ES2724991B2 - METHOD OF OBTAINING AND AUTOMATIC ANALYSIS OF FIELD DATA AND VALIDATION AND / OR CALIBRATION OF SATELLITE PRODUCTS BY MEANS OF SUCH FIELD DATA - Google Patents

Description

DESCRIPTION

METHOD OF OBTAINING AND AUTOMATIC ANALYSIS OF FIELD DATA AND

VALIDATION AND / OR CALIBRATION OF SATELLITE PRODUCTS THROUGH SUCH

FIELD DATA

Technical field

The present invention concerns a method of obtaining and automatic analysis of field data and of validation and / or calibration of satellite products by means of said field data, the field data being atmospheric parameters, including surface temperature, obtained from calculations performed on radiometric field measurements obtained in a region of the earth's surface by means of an automatic measurement station that includes a field radiometer with thermal infrared bands, and the satellite products being atmospheric parameters, which include the earth's surface temperature, obtained from calculations carried out on radiometric satellite measurements, obtained by means of a satellite radiometer with thermal infrared bands, of the same region of the earth's surface.

That is, based on field data measurements taken in a region of the earth's surface, a satellite radiometer integrated in a satellite that takes satellite data from that same region of the earth's surface can be calibrated, and / or satellite products can be validated. obtained by means of algorithms that make use of these satellite data, establishing the value of the uncertainty of said satellite products.

State of the art

Goniometers are often used to take radiometric measurements, but these are bulky instruments that are tedious to handle, so they cannot be used for autonomous and continuous acquisition in the field.

A goniometer is known that allows measurements of a sample under different observation angles, however its handling is also manual and very cumbersome, so it cannot be considered as autonomous and it cannot be permanently located in the field as it requires constant supervision.

A similar goniometer is known from the publication in Sandmeier & Itten (2009, IEEE Transactions on Geoscience and Remote Sensing, vol. 37), which also has dimensions of 4m x 2m, making it difficult to install in certain locations. remote, being also a device without autonomous capacity.

There are also solar trackers that allow a zenith tracking of direct solar irradiance, although these have the motor axis oriented with an angle equal to the latitude in which it is installed and measure under certain celestial zenith angles in that plane. These systems do not allow a complete scan of the celestial and terrestrial hemispheres and do not contemplate the installation of a thermal radiometer in them.

Different companies market them, but they do not have systems similar to the one proposed in this patent for measurement by means of angular thermal radiometry.

The publication Remote Sens. 2015, 7, 15269-15294 “An Autonomous System to Take Angular Thermal-Infrared Measurements for Validating Satellite Products” by the authors Raquel Niclos, José A. Valiente, Maria J. Barbera and César Coll.

This document describes an automatic and mobile measurement station capable of taking radiometric measurements at any zenith and azimuth angular position using a single radiometer.

However, the solution described in this document offers angular radiometric data with a margin of error because the zenith angular position of the field radiometer is not sufficiently accurate (as indicated on page 5). The uncertainties in the angular positioning of said solution lead, for example, to uncertainties in the calculation of the amount of precipitable water vapor, obtained from the readings of said field radiometer, of up to 0.5 cm.

The present invention represents an improvement of the solution described in that document, for example improving the angular positioning, which entails being able to reach uncertainties in the amount of precipitable water vapor of 0.1 cm due to said positioning.

Other publications are also known that propose calculation methods and systems for obtaining the amount of precipitable water vapor (TPW), equivalent to total column water vapor, TCWV) to be used in a calibration or validation of satellite data, but all of them suffer from several deficiencies, such as that they additionally require a reflective diffuser plate bathed in gold and therefore high cost, which do not allow a continuous reading of said atmospheric parameter because they are manual measurement techniques, that do not allow obtaining said atmospheric parameter by means of angular measurements of a single measuring radiometer in the thermal infrared (TIR) spectral region because they measure direct solar irradiance in the near infrared or because they use multiple radiometers that can provide different measurements, which do not allow obtaining said atmospheric parameter at night because they measure in the solar spectrum, or which do not allow deducing the total absence of clouds in the celestial hemisphere from the field TIR radiometer readings because they only perform measured in the direction of solar illumination. A list of antecedents that entail all these deficiencies outlined here would be the following most representative publications:

TITLE: “Estimation of atmospheric water vapor content from direct measurements of radiance in the thermal infrared region”. REF. MAGAZINE / BOOK: “Remote Sensing Letters. Volume 3, 2012 - Issue 1. Pages 31-38 ”. AUTHORS / AS: Vicente García-Santos, Joan Miquel Galve, Enric Valor, Vicente Caselles & César Coll.

TITLE: “Columnar water vapor retrivals from multifilter rotating shadow and radiometer data”. REF. MAGAZINE / BOOK: "Journal of Geophysical Research, 114 (2009)". AUTHORS / AS: Alexandrov, M.D., Schmid, B., Turner, D.D., Cairns, B., Oinas, V., Lacis, A.A., Gutman, S, I., Wstwater E.R., Smirnov, A., Eilers, J.

TITLE: “Water vapor column abundance retrievals during FIFE”. REF. MAGAZINE / BOOK: "Journal of Geophysical Research" Atmospheres, 1992, 97 (D17), 18759-18768. AUTHORS / AS: Carol J. Bruegge, James E. Conel, Robert O. Green, Jack S. Margolis, Ronald G. Holm, Geoff Toon.

TITLE: “Precipitable water estimation from high-resolution split window radiance measurements”, REF. MAGAZINE / BOOK: J. Appl. Meteor., 29, 851-865. AUTHORS / AS: Jedlovec G. J. (1990).

Brief description of the invention

The present invention concerns, according to a first aspect thereof, a method of obtaining and automatic analysis of field data and of validation and / or calibration of satellite products by means of said field data.

Field data are those data related to atmospheric parameters obtained directly or indirectly by an automatic measurement station located in a region of the earth's surface that includes a field radiometer with thermal infrared bands that obtains radiometric field measurements.

Satellite products are data related to atmospheric parameters derived from satellite radiometric measurements obtained by satellites equipped with satellite radiometers with thermal infrared bands.

The validation of these satellite products consists of verifying the precision of these satellite products by comparing them with field data, being the satellite products and the compared field data obtained in relation to the same terrestrial region, allowing to establish error margins of the products. satellites analyzed. The calibration of these satellite products consists of producing calibration equations applicable to the satellite data for the correction of systematic and / or random errors in the measurements obtained by the satellite radiometer, these equations being produced based on the validation analysis of the products. satellite.

It will be understood that a radiometer is a sensor based on obtaining data in spectral bands in the thermal infrared (TIR). Such radiometers typically have a field of view or cone of vision that delimits the region from which they capture information.

The treatment by algorithms of both field data and satellite products allows obtaining atmospheric parameters of the region of the earth's surface analyzed.

The proposed method consists of the following stages, in themselves they are known:

a) obtaining radiometric satellite measurements relative to a region of the earth's surface, by means of at least one satellite equipped with a satellite radiometer with thermal infrared bands;

b) acquisition of radiometric field measurements simultaneous to satellite radiometric measurements and relative to that same region of the earth's surface by means of at least one automatic measurement station located in said region of the earth's surface, said station including a field radiometer with bands in the thermal infrared that obtains independent radiometric measurements in different azimuthal angular positions in 360 ° around, and in different zenith angular positions in vertical planes;

c) calculation, by means of at least one calculation device, of atmospheric parameters that include the earth's surface temperature, calculated from the radiometric satellite measurements obtained from stage a) producing satellite products, and calculated from the radiometric measurements of field obtained from step b) producing field data;

d) comparison between the earth's surface temperature calculated from satellite radiometric measurements and the earth's surface temperature calculated from starting from the radiometric field measurements obtained in step c), validating the calculations for obtaining the satellite products and / or detecting discrepancies between the satellite products and the field data;

e) calculate, by means of the calculation device that is at least one, calibration equations applicable to satellite products in terms of radiance at the roof of the atmosphere for the correction of systematic and / or random errors in satellite radiometric measurements and / or margins of error for satellite products; The aforementioned automatic measurement station will remain fixed at a point during the taking of measurements, and will include a field radiometer that will be used to obtain multiple radiometric measurements that can be used for calculating field data, each radiometric measurement corresponding to a specific azimuth position. and to a specific zenith position of said field radiometer. Therefore, each radiometric measurement obtained will correspond to a region of the celestial or terrestrial hemisphere visible from the position of the automatic measurement station, and preferably together they will provide field data of the entire celestial and terrestrial hemisphere visible from said position.

In order to ensure that all field measurements from the measurement station are performed under identical circumstances, the measurement station has a single field radiometer that moves achieving different azimuth and zenith angular positions to collect each of the multiple data field. If multiple field radiometers were used, there would be a risk that they would have not perfectly comparable responses as a consequence of the inherent uncertainties associated with their different calibration curves.

It is also advisable to locate the automatic measurement station in a thermally homogeneous region of the earth's surface, that is to say that around that station the terrain has conditions as regular as possible, for example monocultures or homogeneous natural landscapes.

For comparison, it is also necessary that the satellite radiometric measurements and the field radiometric measurements are obtained simultaneously, that is, both within a time frame during which the thermal conditions do not vary, for example within a time frame of ten minutes. .

To achieve said displacement of the field radiometer, it is proposed that the field radiometer be hingedly attached to a rotating support which in turn is hingedly attached to a fixed support, the rotating support and the field radiometer being actuated. by means of a zenithal servomotor and an azimuth servomotor controlled by a control device configured to operate said zenith and azimuth servomotors to position the rotary support and the field radiometer in said different zenith angular positions and in different azimuthal angular positions, to obtain data field.

The zenith servomotor will cause a vertical rotation of the field radiometer, modifying its zenith angular position, while the azimuth servomotor will produce a rotation of the radiometer in a horizontal plane, modifying its azimuthal angular position.

Typically the rotating support will be attached to the fixed support by means of a vertical axis shaft, allowing its rotation around a horizontal plane actuated by the azimuth servomotor. In turn, the field radiometer will be attached to the rotating support by means of a horizontal axis shaft, allowing its rotation around a vertical plane activated by the zenith servomotor, although an inverse construction is also plausible.

This automatic station will allow obtaining field data from the entire celestial and also terrestrial hemisphere from which the terrestrial surface temperature can be calculated in the region of the earth's surface where it is located, as well as other atmospheric parameters.

The present method also proposes, in an unknown way, the following steps:

• detect zenith angular position data from the field radiometer by means of a zenith angular position sensor in each of the zenith angular positions of step b);

• perform the zenith angular positioning of the field radiometer in each of the zenith angular positions of stage b) by means of a precise actuation of the zenith servomotor with control commands issued by the control device corrected with the zenith angular positioning data obtained from the zenith angular position sensor;

• calculate, by means of said at least one calculation device, the amount of precipitable water vapor contained in the atmosphere in said region of the earth's surface from the analysis of the variation of the different radiometric field measurements obtained by the field radiometer in different zenith angular positions and azimuthal angular positions within the celestial hemisphere in the absence of clouds;

• introduce said quantity of precipitable water vapor calculated in the calculation of the temperature of the earth's surface in step c).

The detection of the zenith angular position of the field radiometer allows the control device to rectify the control commands of the zenith servomotor to ensure a much more precise positioning of the field radiometer, improving possible deviations. The accuracy of the zenith angular position of the field radiometer is essential to ensure the subsequent use of the field data obtained in the process of cloud detection and calculation of precipitable water vapor.

It has been verified that on average, the correction of the proposed zenith angular position allows reducing the error in the zenith angular positioning of the field radiometer from approximately 0.6 ° on average, and with maximum differences of up to 2 °, as has been obtained during the previous state and also observing certain diurnal drifts, up to an error of approximately 0.3 ° in a more constant way, as is obtained with the application of the present invention. This improvement in the zenith positioning allows to reach uncertainties in the calculation of the amount of precipitable water vapor, TPW, of up to ± 0.1 cm.

The automation of the measurement station allows continuous obtaining of field data, and also the improvement of the zenith angular precision of the radiometric field measurements obtained allows said radiometric field measurements to be used for the precise calculation of the amount of Precipitable water vapor contained in the atmosphere in conditions of absence of cloudiness both during the daytime and during the nighttime period, instead of having only specific measurements currently obtained with other methodologies.

The proposed method for calculating the amount of precipitable water vapor from the radiometric field measurements obtained with the field radiometer is effective under cloudless sky conditions, and is based on the following expressions:

^{L 1 atm (d, $) = L i 1 atm (0) * L i * atm (0 o} ) co s ^{- x (0)}

r \

L ^{1 atm, hem = ------------} L ^{¿^ atm} ( 0 ^O )

2 - x

The first equation relates the descending atmospheric radiance Lhtm ( d, Q) in the thermal infrared for a specific direction of the celestial hemisphere with the radiance at the zenith, Lhtm ( 0 °); while the second equation relates the hemispheric irradiance (integration of the Lhtm ( Q, Q) in the celestial hemisphere) divided by n, Lhrnhem, with the radiance Lhtm ( 0 °).

These expressions are only valid in the case of plane parallel atmospheres, which occur in cloudy skies.

An adjustment on the first equation using the field data obtained by the field radiometer, that is, between ln ( Lh ^tm ( 9)) and ln ( cos ( 9)) for each complete scan of the measurement station, would allow to obtain the parameter xi, dependent on the spectral band of measurement and the atmospheric conditions.

Said parameter xi is dependent on the measurement spectral band and atmospheric conditions, mainly on the amount of precipitable water vapor, since water vapor is the maximum atmospheric absorber in the thermal infrared.

To estimate the amount of precipitable water vapor, a 2 quadratic regression has been established between the amount of precipitable water vapor and the term ------- 2 - x; using the atmospheric variables L ^{^ atm} ( 0 °) and L ^{^ atm, heme} simulated with a model of radiative transfer through the atmosphere and a wide database of atmospheric profiles representative worldwide, so that the regression obtained has global validity and thus can be used for any location of the angle device. This regression will give us the following method for estimating the amount of precipitable water vapor (TPW):

_ ± _ v

TPW (em) = ^c r

2 ci ~ Z ------- co

V 2 - X and 2 - x.

Where the c terms are the numerical coefficients obtained by the aforementioned quadratic regression.

two

Thus, obtaining xi, and therefore the term ^{2 - x} i , from the field data at different angular positions measured by the field radiometer in the celestial hemisphere, the quantity of precipitable water vapor is determined with the previous expression.

The calculation device also allows detecting the absence of clouds in the celestial hemisphere by calculating a value relative to the presence of clouds, and its evaluation below a pre-established threshold, based on the analysis of the variation of the different radiometric measurements of field obtained by the field radiometer at different Zenith angular positions and azimuthal angular positions within the celestial hemisphere.

In the case of partial presence of clouds in the celestial hemisphere, the linear regression between ln ( L 1 ^atm ( 0)) and ln (cos (0)) allows obtaining a correlation coefficient r2 lower than that corresponding to the absence of clouds. .

This method of cloud detection uses this correlation coefficient r2 on which to establish a limit value for the case of clear skies. The limit value in r2 established for clear skies in the proposed cloud detection technique would be 0.9, below this value it would not be possible to calculate the amount of precipitable water vapor, and during these periods it would not be calculated until the sky conditions without cloud cover will be restored.

Thus, the treatment of the field data, which is carried out by at least one calculation device, allows the calculation of the earth's surface temperature, the amount of precipitable water vapor when there are no clouds, and even optionally also allows the presence of clouds to be automatically detected all of them atmospheric parameters relative to the region of the earth's surface where the automatic measurement station is located. Likewise, from the terrestrial surface temperature data obtained from field radiometric measurements, together with the estimated amount of precipitable water vapor, it will be possible to obtain an estimate of the radiance on the roof of the atmosphere measurable with the satellite radiometer over the region of the earth's surface.

The calculation device will compare the field data calculated from the radiometric field measurements with the satellite products calculated from the radiometric satellite measurements. From the differences detected between these values, systematic and / or random errors can be deduced in the measurements obtained by the satellite radiometer, both in terms of radiance at the roof of the atmosphere, in the calibration method, and in terms of temperature of the land area, in the validation method. Calibration equations applicable to satellite products can be calculated to correct such systematic and / or random errors, thus improving the precision of satellite products. Said differences detected also make it possible to detect the existing margins of error in satellite products, thus achieving a validation or certification of the accuracy of said satellite products.

According to an embodiment of the invention the angular separation between the different zenith angular positions and / or between the different azimuthal angular positions in which field measurements performed by the field radiometer can be initially selected by the user, so that both the terrestrial and celestial hemispheres are adequately covered, and can be uniform.

Optionally it is proposed that the angular separation between the different zenith angular positions and / or between the different azimuthal angular positions in which the radiometer readings are made is equal to or less than the width of the radiometer's cone of view, for example being comprised between 15 ° and 22.5 °.

In a preferred example the total number of zenith angle positions will be twelve with selectable angular spacing, while the number of azimuthal positions will be ten with equidistant angle spacings.

It is proposed that the acquisition of the field data at all the zenith and azimuth angular positions be performed together in less than 15 minutes.

Preferably, between readings of the field radiometer at different consecutive zenith and / or azimuthal angular positions pass at least 2 seconds, and preferably 4 seconds, allowing time for the field radiometer to stabilize after its movement. It is also contemplated that the precise zenith and azimuth angular positioning of the radiometer is carried out through the steps of:

• actuate the zenith servomotor controlled by the control device to position the field radiometer in a preset zenith angular position;

• detect zenith angular position data from the field radiometer using an angular position sensor configured to detect a zenith angular position of the field radiometer relative to vertical;

• communicate the detected zenith angular position to the control device,

• detect, by said control device and by analyzing the detected zenith angular position, angular deviations of the field radiometer with respect to the preset zenith angular position;

• actuate the zenith servomotor controlled by the control device to perform a correction on the zenith angular position of the field radiometer, correcting by the detected deviations.

When a deviation is detected, the position of the field radiometer can be corrected immediately before taking the next radiometric field measurement, or the position can be corrected after taking the next radiometric field measurement at the perform the next field radiometer shift, including correction on that shift.

According to a second aspect of the invention, which has not been claimed, it concerns a system for obtaining and automatic analysis of field data and for the validation and / or calibration of satellite products by means of said field data.

The proposed system includes:

• at least one automatic measurement station located in a region of the earth's surface that includes a field radiometer with thermal infrared bands that obtains independent field radiometric measurements at different azimuth angular positions in 360 ° around, and at different Zenith angular positions in vertical planes, to obtain field data;

• at least one calculation device configured to calculate atmospheric parameters that include the Earth's surface temperature from radiometric field measurements, producing field data, and to compare those field data with satellite products that include the calculated Earth's surface temperature from satellite radiometric measurements, for the validation and / or calibration of satellite products;

wherein the automatic measurement station includes the field radiometer hingedly attached to a rotating support which in turn is hingedly attached to a fixed support, the rotary support and the field radiometer being actuated by a zenith servomotor and a Azimuthal servomotor controlled by a control device configured to operate said zenith and azimuth servomotors to position the rotary support and the field radiometer in said different zenith angular positions and in different azimuthal angular positions, for obtaining field data.

The assessments made in relation to the devices and elements described in the method are equally applicable to the present system.

The system further proposes that the field data acquisition automatic measurement station include a zenith angular position sensor configured to detect a zenith angular position of the field radiometer and to communicate the detected zenith angular position to the control device. The control device will be configured for the precise correction of said zenith angular position of the field radiometer in response to the zenith angular position detected by the control device by means of the precise actuation of at least the zenith servomotor.

Preferably the zenith angular position sensor will be a two-axis accelerometer linked to the field radiometer for the detection of its position with respect to the vertical defined by gravity.

Other characteristics of the invention will appear in the following detailed description of an embodiment.

Brief description of the figures

The foregoing and other advantages and characteristics will be more fully understood from the following detailed description of an embodiment with reference to the attached drawings, which should be taken by way of illustration and not limitation, in which:

Fig. 1 shows a perspective view from below of the automatic metering station; Fig. 2 shows a perspective view from above of the same automatic measuring station shown in Fig. 1, in which arrows indicative of the rotational movements that the different parts of the same can perform have been included;

Fig. 3 shows a graph in which the deviation of the zenith angular position of the field radiometer in degrees with respect to the theoretical position in which it should be positioned is shown on the ordinate axis, and different dates on the abscissa axis measurement, corresponding to an automatic measurement station in which a correction of the zenith angular position of the field radiometer is not carried out based on the readings of the zenith angular position sensor;

Fig. 4 shows the same graph but corresponding to an automatic measurement station in which a correction of the zenith angular position of the field radiometer is performed based on the readings of the zenith angular position sensor.

Detailed description of a realization example

The attached figures show exemplary non-limiting illustrative embodiments of the present invention that concerns a method for obtaining and automatic analysis of field data and for the validation and / or calibration of satellite products using said field data.

A satellite (not shown) equipped with a satellite radiometer measuring bands in the thermal infrared, in the spectral region between 8 and 14 pm, with observation over a thermally homogeneous region of the earth's surface obtains radiometric measurements satellite from which satellite products are obtained by calculation. The system also includes an automatic measurement station located in said region of the earth's surface that includes a field radiometer 10 with measurement in thermal infrared bands, in the spectral region between 8 and 14 pm, which obtains independent radiometric measurements in different Azimuthal angular positions in 360 ° around, and in different zenith angular positions in vertical planes, for obtaining field data.

Such an automatic measurement station can automatically rotate to obtain a set of zenith and azimuth viewing angles for which the field radiometer 10 takes measurements.

The advantage of this configuration is to perform full scans of the terrestrial and celestial hemispheres using scan measurements at predefined angles. Furthermore, the use of a single field radiometer 10 with a single calibration avoids potential inter-calibration problems.

According to an example of embodiment shown in Fig. 1, the proposed automatic measuring station has a rotating support 20 in the form of a disk that rotates around a fixed support 30 in the form of a hollow cylinder, which simultaneously acts as a shaft. vertical axis of the system, to which it is connected by a ball bearing. This configuration allows rotary mount 20 to rotate around fixed mount 30 to achieve any desired azimuth angular position. An azimuth servomotor SA fixed below the rotary support 20 and a gear wheel concentric to the hollow cylinder and fixed thereto are the mechanical parts that are in charge of driving the system to the selected azimuth angular positions. A reverse construction is also contemplated in which the sprocket is attached to the rotary mount 20 and the azimuth servomotor is attached to the hollow cylinder.

The hollow cylinder allows the insertion of a 40mm diameter pole of any desired height for taking measurements. Many conventional weather station towers end in a mast to accommodate different sensors. The system can be integrated into such towers at any desired level, and not necessarily the highest.

A zenith servomotor SC is fixed to the lower face of the rotary support 20, its shaft being in a horizontal and radial position with respect to the azimuth axis shaft defined by the hollow cylinder of the fixed support 30, thus constituting a zenith axis shaft.

A field radiometer 10 with detection of bands in the thermal infrared is attached to said zenith axis shaft of the zenith servomotor SC, the field of view thereof being perpendicular to said zenith axis tree.

By means of this arrangement, the actuation of the azimuthal servomotor SA modifies the azimuth angular position of the field radiometer 10, and the actuation of the zenith servomotor SC modifies its zenith angular position, thus achieving altogether any azimuth and zenith angular position, allowing the radiometer to field 10 captures information from any region of the terrestrial or celestial hemisphere.

The field radiometer 10 is preferably enclosed within a rectangular shaped head made of polyamide, which is externally insulated with a foam film covered with an aluminum foil. The head is fixed to the zenith axis shaft of the zenith servomotor SC and acts as a radiant shield to avoid possible heating of the field radiometer 10 with the sun.

Said head also contains the zenith angular position sensor 11 which provides a precise knowledge of the zenith angle of the field radiometer 10. Said zenith angular position sensor 11 can be for example a 2D or two-axis accelerometer.

Said zenith angular position sensor 11 makes it possible to check that the zenith angular position of the field radiometer 10 actually achieved corresponds to the predicted zenith angular position. The accuracy of this position is important to ensure that the atmospheric parameters calculated from the field data are also accurate.

From the readings of the zenith angular position sensor 11, the central angular positioning of the field radiometer 10 can be corrected by applying the correction algorithm described above.

An experimental comparison between the positioning error made before the introduction of the improvement and after it, the difference between the ordered positioning and the one that is actually achieved, is shown in Figures 3 and 4. In them it can be seen as the error in the zenith angular position of the field radiometer 10 frequently exceeds 0.4 ° and how it can reach 2 °, and as the introduction of the zenith angular position sensor 11 and the correction of the zenith angular position made thanks to the Measurements of this sensor allow the zenith angular position error to be reduced to below 0.4 ° in practically all the measurements made.

The inclusion of an azimuth angular position sensor is also proposed that provides a precise knowledge of the azimuth angle of the field radiometer. Said azimuth angular position sensor can be for example a magnet attached to the underside of the rotary support in combination with three equidistant reed switches attached to the hollow cylinder.

The actuation of the azimuth and zenith servomotors to position the field radiometer in the different azimuthal and zenith angular positions is controlled by a control device or processor with data logging.

Preferably between the control device and the zenith SC and azimuth SA servo motors a serial servo-controller is inserted as an interface or command translator in order to achieve a precise positioning and smooth movements of the azimuth and zenith servo motors.

The field radiometer 10 can be located in any azimuthal and zenith angular position, however to speed up the readings the number of azimuth and zenith angular positions can be limited and still obtain data from the entire terrestrial and / or celestial hemisphere, depending on the aperture of the field of view of the field radiometer. The greater the opening of the field of view, the fewer readings will be necessary to cover the entire hemisphere, to the detriment of the angular resolution of the entire set of measurements.

In the present case the system is operatively configured to obtain readings from the field radiometer 10 at ten different azimuthal angular positions, corresponding to 18 °, 54 °, 90 °, 126 °, 162 °, 198 °, 234 °, 270 °, 306 ° and 342 ° from north, and at twelve zenith angular positions with selected spacing, corresponding to 0 °, 18 °, 36 °, 54 °, 72 °, 90 °, 112 °, 122 °, 142 °, 152 ° , 158 ° and 180 ° where 0 ° is the vertical zenith direction and 180 ° is the vertical nadir direction. For this configuration, a full sweep of both hemispheres, sky and earth, fits well within a 15 minute period.

The system takes the twelve consecutive measurements at the different zenith angular positions and at the same azimuth angular position, moves to the next azimuth angular position through a horizontal rotation of the rotary holder 20, and begins taking the next twelve measurements at the different zenith angular positions selected.

The period of time between one measurement and the next is fixed at four seconds. During the first second, the control device commands the zenith servomotor SC to actuate it to produce the change in zenith angular position of the field radiometer 10. After the next two seconds, the control device proceeds to compare the preset position to the one that the field radiometer 10 had to be positioned after activating the zenithal servomotor SC with the data from the zenith angular position sensors 11. The existing differences will be taken into account in the next position for the same vertical position and the same direction of rotation , thus achieving progressive correction and more precise positioning of the field radiometer 10. These two seconds also allow the field radiometer 10 to stabilize after its displacement. Finally, during the fourth second, the measurement is carried out by the field radiometer 10.

The field radiometer used is a thermal radiometer between 8-13 pm that has an accuracy of ± 0.2K in temperatures between 278K and 303K, and that must have a maximum cone of vision of 36 °.

Preferably the region of the earth's surface where the automatic measuring station is located will be a flat and thermally homogeneous region, for example a monoculture area, a desert or semi-desert area, a wetland area, a swampy area, a lake, a saline, or similar that provides similar emissive properties, and with a temperature as uniform as possible in an area of at least 3x3 km2.

The field data obtained from the field radiometer 10 is processed by a computing device that receives the field data from the field radiometer 10 and also the satellite data obtained by the satellite radiometer.

The radiance measured in an angular direction Q azimuthal and zenith 0 by a radiometer field 10 in the band i observing an area from the ground level (L i (Q, 0)) can be expressed as follows:

Lt ( O , <P) = * t. ( 6 >, <P) B, ( T) (1 - ( 6 >. 0)) Lt \ mj, m

In this equation the parameter e¡ ( Q, 0) is the directional surface emissivity, B¡ ( T) is the mean of the Planck function in band i, T is the mean surface temperature, and Lhtm, hem is the radiance descending atmospheric.

The parameter L ^ ^{atm, hem} can be obtained through the integration of the value of atmospheric radiance from a given angular direction Lxatm ( Q, Q).

In the case of horizontal homogeneity of the atmosphere, that is, in conditions of total absence of clouds, or of uniform cloud cover, the atmospheric radiance from a given angular direction L¡iatm ( 8, Q) follows the following expressions:

^{L 1 atm (0, 0) = L i 1 atm (0) * L. 1 atm} (0o) co sx ^{( 0 )}

r \

L ^ atm, hem = ------------ L ¿^ atm ( 0 O)

2 - x

The condition of total absence of clouds is necessary to obtain the Earth's surface temperature (LST) from the satellite data obtained by the satellite radiometer, therefore, to validate said satellite data, the Earth's surface temperature must be obtained. in conditions of absence of cloudiness from the field radiometer.

A fit on the first equation using the field data obtained by the

field radiometer, that is, between ln ( L¡ ^ atm ( 0)) and ln (cos (0)) for each scan

complete measurement station, would allow to obtain the parameter X, dependent on the spectral band of measurement and atmospheric conditions.

two

Since the parameter X ⁱ , and therefore the term ------- in the second equation shown, is 2 - xi

obtained from the field data at different angular positions measured by the field radiometer, the amount of precipitable water vapor TPW, as an atmospheric parameter, can be estimated from the following equation:

To obtain this equation, a quadratic regression has been established between the quantity

of precipitable water vapor and term ² using the atmospheric variables Lhtm 2 - x

( 0 °) and Lhmhem simulated with a model of radiative transfer through the atmosphere and an extensive database of globally representative atmospheric profiles, in a that the regression obtained has global validity, that is, that it can be used for any location of the angular device. The c terms of the equation are the numerical coefficients obtained with the regression. The established regression could also be linear rather than quadratic.

In the case that the land surface region has a thermohomogeneous and isotropic surface cover (such as water surfaces or certain monocultures, such as rice fields), in which the land surface temperature does not depend on observation angles, the evaluation of the emissivity values relative to the nadir (opposite to the zenith), avoids the knowledge of the Earth's surface temperature itself by taking two measurements of radiance, one at the nadir, that is (0,0), and the other in a defined angular configuration (0 , 0), using the following expression described in detail in the following post:

Angular variation of land surface spectral emissivity in the thermal infrared: Laboratory investigations on bare soils. Int. J. Remote Sens. 1991, 12, 2299-2310. "

Since the proposed system measures the radiances in a set of azimuthal and zenith observation angles, the emissivities relative to the nadir can be determined with the previous equation to analyze the anisotropy of the emissivities for the homogeneous surfaces observed by the system.

The treatment of field data by applying the calculations described above is carried out automatically by at least one calculation device, and allows obtaining parameters that can be used to validate or correct the satellite data obtained by the satellite radiometer. .

Such satellite data typically include roof-of-the-atmosphere (TOA) radiances for spectral bands from mid-wave infrared to thermal infrared (for example, at 3.7, 10.85, and 12.0 pm) and at the case of sensors with dual vision, both for the nadir and for an oblique view (around a 55 ° zenith angle).

The radiances in the roof of the atmosphere TOA in the spectral bands at 11 and 12 pm ( L ^{í, toa} ( 9, $)), or the equivalent brightness temperatures in these bands, and in the views nadir and oblique, are used to obtain the terrestrial surface temperature by means of algorithms or techniques of differential absorption in the atmosphere, of the "split-window" or "dualangle" type, which allow correcting the atmospheric effect while correcting the effect of surface emissivity. These techniques usually require previous estimates of the surface emissivity for the direction of vision by the satellite, e, (0,0), and of the precipitable water vapor contained in the atmosphere, TPW; parameters obtained from the field data measured by the system. Thus, the validation of the terrestrial surface temperature obtained from the satellite data (using these emissivity and TPW field data required in the corrections of the satellite data) versus the terrestrial surface temperature obtained with the field data does not require complementary data and allows estimating the degree of deviation of said satellite products, validating their degree of uncertainty. Since the field measurement device allows angular data to be obtained, the validation of satellite products can be refined using field data measured with observation angles similar to the viewing angles by the satellite.

Claims (10)

1. Method of obtaining and automatic analysis of field data and of validation and / or calibration of satellite products using said field data, which consists of the following stages: a) obtaining radiometric satellite measurements relative to a region of the earth's surface, by means of at least one satellite equipped with a satellite radiometer with thermal infrared bands; b) acquisition of radiometric field measurements simultaneously with satellite radiometric measurements and relative to that same region of the earth's surface by at least one automatic measurement station located in said region of the earth's surface, said measurement station including a field radiometer (10) with bands in the thermal infrared that obtains independent radiometric measurements in different azimuthal angular positions in 360 ° around, and in different zenith angular positions in vertical planes; c) calculation, by means of at least one device for calculating atmospheric parameters that include the earth's surface temperature, calculated from the radiometric satellite measurements obtained from step a) producing satellite products, and calculated from the radiometric field measurements obtained from step b) producing field data; d) comparison between the terrestrial surface temperature calculated from the satellite radiometric measurements and the terrestrial surface temperature calculated from the radiometric field measurements obtained in step c), validating the calculations for obtaining the satellite products and / or detecting discrepancies between satellite products and field data; e) calculate, by means of said at least one calculation device, calibration equations applicable to satellite products in terms of radiance at the roof of the atmosphere for the correction of systematic and / or random errors in satellite radiometric measurements and / or margins of error for satellite products; characterized in that the method also includes the following steps: detecting zenith angular position data from the field radiometer (10) by means of a zenith angular position sensor (11) in each of the zenith angular positions of step b); perform the zenith angular positioning of the field radiometer (10) in each of the zenith angular positions of stage b) by means of a precise actuation of a zenith servomotor (SC) with control commands issued by a control device corrected with the data of the previous zenith angular positioning obtained from the zenith angular position sensor (11); calculate, by means of said at least one calculation device, the amount of precipitable water vapor contained in the atmosphere in said region of the earth's surface from the analysis of the variation of the different readings obtained by the field radiometer (10) in different zenith angular positions and azimuthal angular positions within the celestial hemisphere in conditions of absence of clouds; and use said amount of precipitable water vapor calculated in the calculation of the terrestrial surface temperatures in step c). 2. Method according to claim 1 wherein the calculation device detects the absence of clouds in the celestial hemisphere by calculating a value relative to the presence of clouds in the atmosphere in said region of the earth's surface lower than a pre-established threshold from of the analysis of the angular variation of the different readings obtained by the field radiometer (10) in different zenith angular positions and azimuthal angular positions within the celestial hemisphere. 3. Method according to claim 1 or 2 wherein the method is applied both day and night. 4. Method according to claim 1, 2 or 3 wherein the angular separation between each of the different zenith angular positions and / or between each of the different azimuthal angular positions in which the field radiometer readings (10) are made is a uniform angular separation. 5. Method according to any one of the preceding claims, wherein the angular separation between the different zenith angular positions and / or between the different azimuthal angular positions in which the readings of the field radiometer (10) are made is equal to or less than the width of the cone of vision of the field radiometer (10). Method according to any one of the preceding claims, wherein the total number of zenith angular positions is twelve with selectable angular separation while the number of azimuthal positions is ten with equidistant angular separation. 7. Method according to any one of the preceding claims, wherein the acquisition of the field data in all zenith and azimuthal angular positions is carried out in less than 15 minutes. Method according to any one of the preceding claims, wherein between readings of the field radiometer (10) at different consecutive zenith and / or azimuthal angular positions pass at least 3 seconds. Method according to any one of the preceding claims, wherein the earth's surface temperature, the absence of clouds and the amount of precipitable water vapor in the atmosphere are calculated by the calculation device by applying algorithms to the data obtained by the field radiometer (10). Method according to any one of the preceding claims, wherein the precise zenith and azimuth angular positioning of the field radiometer (10) is carried out by means of the steps of: • actuating the zenith servomotor (SC) controlled by the control device to position the field radiometer (10) in a pre-established zenith angular position; • detecting zenith angular position data from the field radiometer (10) by means of a zenith angular position sensor (11) configured to detect a zenith angular position of the field radiometer (10); • communicate the detected zenith angular position to the control device, • detecting, by said control device and by analyzing the detected zenith angular position, angular deviations of the field radiometer (10) with respect to the pre-established zenith angular position; • actuation of the zenith servomotor (SC) controlled by the control device to make a correction on the next zenith angular position of the field radiometer (10) correcting by the detected deviations.